Why are my fish dying? Tropical fish tank.?

My tank is 3 months old and for 2 months I only had the starter fish in it (2 mollys 4 platy). 1 month ago I added 8 small neon tetras and 4 guppys (water test was fine). Now my fish are dying daily (old and new). I have been doing regular water changes and feeding every other day. I had the water tested and it is fine. Temperature is 26C There are no signs of white spot disease and the fish are not gasping for air so why are they dying? PLEASE HELP!
Thanks 4 ur answers and advice..The tank is 40 litres